FRS 6. ANATOMY OF THE SMALL AND LARGE INTESTINES

Question 1

What are the sections of the large intestine?

Answer 1

o caecum
o appendix
o ascending colon, transverse colon, descending colon
o sigmoid colon (then rectum and anal canal)

Question 2

What are the characteristic features of the large intestine?

Answer 2

 Appendices epiploicae – tags of fat (small pouches of peritoneum,
filled with fat) attached to the surface of the large intestine
 Teniae coli – the outer longitudinal muscle is arranged into three
bands of muscle
 Haustra – when the teniae coli contract, the walls of the bowel
shorten to form sacculations known as haustra

Question 3

What is the ileocaecal junction

Answer 3

The large intestine joins the small intestine at the ileocaecal junction
o demarcates the caecum from the ascending colon

Question 4

What is the appendix and where is it located?

Answer 4

also known as the vermiform (worm-shaped) appendix is a narrow, blind ended tube
attached to the posteromedial end of the caecum.
o The position of the free-end of the appendix is highly
variable, and can be categorised into seven main
locations.
o The most common positions are retrocaecal and subileal
o The appendix can be located by looking at the point at
which the 3 bands of taenia coli converge

Question 5

What is the caeum?

Answer 5

 Most proximal part of the large intestine – located between the
ileum and the ascending colon
 Used to serve as a site for cellulose digestion in ancestors but no
real function now apart from acting as a reservoir for chyme that it receives from the ileum

Question 6

Describe the ascending colon

Answer 6

 retroperitoneal; ascends superiorly from the caecum.
 When it meets the right lobe of the liver, it turns 90 degrees to move horizontally
 This turn is known as the right colic flexure (or hepatic flexure); marks the start of the transverse colon.

Question 7

Describe the transverse colon

Answer 7

 crosses the abdomen, extends from the right colic flexure to spleen;
turns 90 degrees to point inferiorly.
 This turn is known as the left colic flexure (or splenic flexure).
 Here, the colon is attached to the diaphragm by the phrenicocolic
ligament.
 The transverse colon is the least fixed part of the colon, and is variable
in position
 Unlike the ascending and descending colons, the transverse colon is
intraperitoneal

Question 8

Describe the descending colon

Answer 8

 After the left colic flexure, the colon moves inferiorly, and so is termed the descending colon
 retroperitoneal in most people, but anterior to the left kidney, passing over its lateral border
 When the colon begins to turn medially, it becomes the sigmoid colon

Question 9

Describe the sigmoid colon

Answer 9

 40cm long; located in theleft lower quadrantof the abdomen, extends from left iliac fossa to level of S3
vertebra
 characteristic “S” shape
 The sigmoid colon is attached to the posterior pelvic wall by a mesentery – thesigmoidmesocolon
 The long length of the mesentery permitsthis part of the colon to be particularly mobile.

Question 10

What are the peritoneal folds

Answer 10

 Only two parts of the colon are supported by peritoneal folds known as the mesentery
o Transverse mesocolon => this structure attaches the colon to the back abdominal wall
o Sigmoid mesocolon
 These carry blood and lymphatic vessels

Question 11

What are the paracolic gutters?

Answer 11

 two recesses between the ascending/descending colon and the posterolateral abdominal wall
 clinically important – allow infective material that has been released from the abdominal organs to
accumulate

Question 12

What are the sections of the small intestine?

Answer 12

• duodenum
• jejunum
• ileum

Question 13

What is the duodenum and what are its sections?

Answer 13

Most proximal portion of the small intestine – it is C-shaped and
adjacent to the pancreas (retroperitoneal)
Divided into four parts which collectively form C-shape:
- superior
- descending
- inferior
- ascending

Question 14

Describe the superior section of the duodenum

Answer 14

SUPERIOR (L1)
 5cm in length
 Ascends upwards from pylorus of the stomach
 Connected to liver by the hepatoduodenal ligament
 The initial 3cm is covered by the visceral peritoneum  remainder is
retroperitoneal

Question 15

Describe the descending section of the duodenum

Answer 15

DESCENDING (L1-3)
 7.5 cm
 Curves around the head of pancreas – lies posterior to transverse colon and
anterior to right kidney
 Contains the major duodenal papilla

Question 16

Describe the inferior section of the duodenum

Answer 16

 10 cm
 Travels laterally to the left
 Crosses over the IVC and AA, located inferior to the pancreas

Question 17

Describe the ascending section of the duodenum

Answer 17

 2.5 cm
 After crossing aorta it ascends and curves anteriorly to join the jejunum at a sharp turn known as the
duodenojejunal flexure
 located at the duodenojejunal junction is a slip of muscle called the suspensory muscle of the duodenum
 contraction widens the angle of the flexure, and aids movement of the intestinal contents into the
jejunum

Question 18

Describe the jejunum

Answer 18

 Represents two fifths of the last two sections of the small intestine (40%)
 Begins at the duodenojejunal flexure
 Mostly located in the left upper quadrant
 larger diameter and thicker wall than ileum
 Inner mucosal lining of the jejunum characterised by presence of plicae
circulares (circular folds) => most pronounced in the jejunum

Question 19

Describe the ileum

Answer 19

 Makes up the distal three-fifths of the small intestine (60%)
 Mostly found in the right lower quadrant
 Has thinner walls and fewer, less prominent plicae circulares
 Has more mesenteric fat and arterial supply
 Opens into the large intestine – where caecum and ascending colon join together
 Two flaps project into the lumen of cecum – called the ileocecal fold flaps
o The folds come together forming muscular ridges – acts as a sphincter to prevent reflux of
material

Question 20

Which sections of the small intestine are intraperitoneal?

Answer 20

both the jejunum and ileum are intraperitoneal as they are both attached to the posterior abdominal wall by
mesentery (a double layer of peritoneum)both the jejunum and ileum are intraperitoneal as they are both attached to the posterior abdominal wall by mesentery (a double layer of peritoneum)

Question 21

Briefly describe the blood supply to the gut

Answer 21

 ascending colon + proximal 2/3 rds of the transverse colon + jejunum and ileum
o derived from the midgut => supplied by the superior mesenteric artery

 distal 1/3 rd of transverse colon + descending colon + sigmoid colon
o derived from the hindgut => these structures are supplied by the inferior mesenteric artery

Question 22

Describe the blood supply to the jejunum and ileum

Answer 22

 Superior mesenteric artery arises from the abdominal aorta, it then gives
off branches to the left supplying the jejunum (located in the upper left
part of the infracolic compartment)
o There are connections formed between the small branches
known as ARTERIAL ARCADES
 There are more branches given off from the superior mesenteric artery
to the ileum  there are MORE ARTERIAL ARCADES in the ileal branches
 From the arcades, long and straight arteries arise, called vasa recta 
jejunum has longer vasa recta and the ileum has shorter vasa recta
 The blood vessels run through the mesentery

Question 23

Describe the blood supply to the ascending colon and proximal 2/3 of the transverse colon

Answer 23

 The superior mesenteric artery also gives rise to blood vessels on the right – a total of three branches of
colic arteries supplying different areas of the colon
o Each colic artery has two branches
 MIDDLE COLIC ARTERY – supplies proximal 2/3 of the transverse colon
 RIGHT COLIC ARTERY - goes towards the hepatic flexure and supplies the ascending colon
 ILEOCOLIC ARTERY - supplies last part of ileum, caecum, and appendix

NB: The middle, right and ileocecal branches anastamose to form the MARGINAL ARTERY; the marginal artery
itself gives of small branches through its length

Question 24

Describe the blood supply to the duodenum

Answer 24

Proximal major duodenal papilla– gastroduodenal artery(branch of thecoeliac trunk).

Distal to major duodenal papilla– inferior pancreaticoduodenal artery(branch ofsuperior mesenteric
artery).

Question 25

Describe the blood supply to the hindgut structures

Answer 25

 descending colon - left colic artery (branch of the inferior mesenteric artery)
 sigmoid colon- sigmoid arteries (branches of the inferior mesenteric artery)
o become the rectal artery

Question 26

What is the lymphatic drainage of the small intestine?

Answer 26

 Duodenum: pancreatoduodenal and superior mesenteric nodes.

 Jejunum and Ileum: superior mesenteric nodes.

Question 27

Describe the lymphatic drainage of the colon

Answer 27

 ascending and transverse colon - superior mesenteric nodes.
 descending colon and sigmoid - inferior mesenteric nodes
 Most of the lymph from the superiormesenteric and inferior mesenteric nodes passes into theintestinal
lymph trunks, and on tothecisterna chyli–where it ultimately emptiesinto the thoracic duct.

Question 28

What is meckel’s diverticulum

Answer 28

EMBRYOLOGICAL ABNORMALITIES
 AMeckel’s diverticulum, a truecongenital diverticulum, is a slight bulge in thesmall intestine present at birth and avestigial remnant of the omphalomesenteric duct(also called the vitelline duct or yolk stalk).
 most common malformation of thegastrointestinal tract and is present in approximately 2% of the population,with males more frequently experiencing symptoms.

Question 29

Describe the histology of the duodenum

Answer 29

The gastroduodenal junction which is at the pyloric sphincter, marks the sharp transition from the
glandular mucosa of the stomach to the villous mucosa of the duodenum
o Note the thickening of muscle at the pyloric sphincter
 The duodenum is distinguished from jejunum and ileum by the presence of brunner’s glands
(predominantly found in the submucosa – poorly stained)
o secrete an alkaline fluid composed of mucin; exerts a physiologic anti-acid function by coating
the duodenal epithelium, protecting it from the acid chyme of the stomach
o also produce lysozyme and EGF

Question 30

What are the 3 stages of folding in the small intestine

Answer 30

• plicae circulares
• villi
• microvilli

Question 31

Describe plicae circulares

Answer 31

large circular folds with a submucosal core

o Muscularis externa lies beneath it – supporting the villi

Question 32

Describe the villi

Answer 32

cover the plicae circulares
o Interspersed with short glands known as crypts of Lieberkühn which extend down to the
muscularis mucosa (project into the lamina propria)
o Villi may contain:
 Blood capillaries
 lacteals
 strands of smooth muscle (from muscularis mucosae)  gives villi contractile element
 This may help squeeze the lymph along lacteals

o NB: the base of villi represents the “true surface” of the small intestinal epithelium

Question 33

Describe the microvilli

Answer 33

found on top of the enterocytes – form the brush border

Question 34

What are Peyer’s patches?

Answer 34

lymphoid aggregates found in submucosa

Question 35

What is the lining of the intestinal villi?

Answer 35

intestinal villi are lined by simple columnar epithelium which is continuous with that in the crypts
The epithelium of the villi is made up of tall columnar absorptive cells calledenterocytes, andgoblet cells,

Lamina propria extends between crypts and into core of each villus – rich vascular and lymphatic network

Question 36

Describe enterocytes

Answer 36

o Most numerous cell type
o Tall columnar cells with microvilli – which forms the brush border (increased surface area)
Brush border formed by microvilli
The goblet cells are poorly stained
 Microvilli are surrounded by a glycocalyx
o absorptive cells
o There are tight junctions between enterocytes

Question 37

What are the additional cells found in the crypts of Lieberkühn?

Answer 37

Paneth cells:
o Found at the base of the crypts; distinguished by their eosinophilic granules (granular cytoplasm)
o Secrete granules of antimicrobial peptides called defensins, lysozyme and phospholipase A

Endocrine cells:
o Produce secretin, somatostatin, enteroglucagon and serotonin

Stem cells:
o Found at base of crypts – replace enterocytes, goblet cells, paneth cells etc.

Question 38

Describe the histological differences throughout the small intestine

Answer 38

1. The villi tend to be longest in the duodenum and shortest in the ileum
2. Lymphoid tissues become more prominent in the ileum (very sparse in the duodenum)
3. Proportion of Goblet cells in epithelium increases distally
4. Plicae circulares are most prominent and numerous in the jejunum – not as prominent in the
   proximal duodenum and distal ileum

Question 39

Describe the histology of the large intestine

Answer 39

 The thickmucosahas deep crypts, but there are no villi
 Epithelium = columnar absorptive cells with a striated border, many goblet cells, endocrine cells and
basal stem cells, but no Paneth cells
 The surface epithelial cells are sloughed into the lumen, and have to be replaced around every 6 days.
 Thelamina propriaandsubmucosaare similar to the small intestine
 Thelongitudinal smooth musclein themuscularis externais arranged in three longitudinal bands
calledtaenia coli
 At the anus, the circular muscle forms the internalanal sphincter.

Question 40

Describe the histology of the appendix

Answer 40

 Small, star-shaped lumen
 Infiltrated with lymphocytes
 arrangement of the layers in its walls is similar to that seen elsewhere in the large intestine
 However, the outer layer of muscle fibres in themuscularis externaforms a continuous layer.
 presence of masses oflymphoid tissuein themucosaandsubmucosa
 There are often follicles containing paler germinal centres, similar to the follicles of Peyers patches